The present invention relates to a method for the generation of timing signals in communication systems and networks and to an apparatus applying this method. The equipment inserted in a synchronous data transport network requires a function providing a timing source for correct operation. For example, the SETS function defined by ITU-T G.783 in the version of August 1997 represents the clock of the SDH network elements.
The timing function includes an internal oscillator function supplying the system clock for all the equipment. The internal oscillator receives a reference clock supplied by a function which generates the timing for the synchronous equipment, for example SETG in SDH systems). The timing is chosen from among a series of possible synchronization sources derived from three possible reference points. These possible reference points are derived from the synchronous input signals, for example, STM-N, from the plesiochronous input signals (for example PDH) or from an external synchronization input signal. In all three cases references are obtained which we shall call t1, t2 and t3 respectively.
The solution of deriving the timing from input signals appears optimal for various reasons. Unfortunately the elements of modern networks, for example digital cross connectors, can have synchronous or plesiochronous interface input signal cards located quite far away and even up to a hundred meters from the central nucleus of the equipment where the timing source function is realized.
There is thus the problem of transporting t1 or t2 from the interface card to the equipment nucleus while keeping the integrity of the signals.
The possible solution for correct transport of t1, t2 over dedicated cables for long distances requires particular treatment of the signals, for example, equalization. In addition, this solution increases the number of cables involved and today the number of cables interconnecting the central nucleus to the peripheral parts is a basic point. Being able to reduce this number makes the equipment simpler to install, more attractive in terms of space occupied and cost and makes possible heat dissipation problems less critical.